Nowadays lots of digital (still and movie) cameras are available on the market. Different kinds of diaphragms for controlling the amount of light falling onto a photodetector are utilized in lens optics. The most widely used diaphragm is the iris diaphragm. However, this mechanical iris diaphragm has a number of disadvantages. It is not suitable for small lens assemblies, as for example in current small digital cameras, due to the necessary size of the mechanical components. In addition, it is not possible to control the shape of the aperture. As springs and a mechanical movement are used, the iris diaphragm needs maintenance. Finally, this type of diaphragm has a relatively slow response.
To circumvent at least some of the above problems, electronic diaphragms based on liquid crystal elements have been proposed. However, liquid crystal elements have a rather low optical transmissivity and are not suitable for all applications.
As a further alternative, a variable iris using small charged opaque particles (electrophoretic particles) has been developed. According to this approach the iris aperture pattern is adjusted by controlling the positions of the opaque particles with electrodes. This type of variable iris has a higher transmissivity of the aperture compared than a liquid crystal type iris.
For example, JP 2004-012906 describes a light control element consisting of electrophoretic particles confined between two substrates. A plurality of ring shaped electrodes is used for moving the particles to a desired location between the substrates. In this way a variable aperture is formed.
Similarly, JP 2004-061832 describes an electrophoretic light quantity adjusting element. Opaque particles confined between two transparent electrodes, which are surrounded by respective opaque electrodes, are used for controlling the quantity of light impinging on a CCD detector. By applying a voltage to the electrodes the particles are moved into or out of the aperture formed by the transparent electrodes.